One of the rarest mental disorders that have been documented and studied is known as the Cotard’s syndrome or the Nihilistic Delusion. Some even refer to it as the Walking Corpse Disease, due to the nature of the illness. People who are affected by this sickness have the delusional belief that they are actually dead & do not exist.
There is another form of the delusional sickness where the person involved believes that they have lost their blood and that their internal organs have disappeared. But an even rarer form of the illness sees patients believe they are immortal.
It comes as no surprise that in most cases the patient is also diagnosed with schizophrenia and have multiple psychotic problems. The disease is named after the 19th-century French neurologist Jules Cotard who first spotted and discovered the illness. He noted that the characteristics of someone suffering from the disease were intense self-loathing and chronic depression. The original case that Cotard studied was a female who eventually died of starvation as she believed she didn’t need to eat and even went so far as to deny some parts of her body existed.
After this point the disease has been intensely studied, however, cases to check theories and confirm why it happens are extremely rare. The general belief is that patients eventually end up in a state where they have a complete distorted view of the world, they become socially withdrawn and tend to completely neglect their own hygiene.
There are a couple of cases of successful treatment so all is not lost if you were to enter this delusional state, but it does require intense therapy and heavy doses of antidepressants. Because the syndrome is thought to act on the areas of the brain that recognize faces, for example, you may see your friend and believe they are an imposter.